Transition metal single-atom embedded on N-doped carbon as a catalyst for peroxymonosulfate activation: A DFT study

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Single-atom catalysts perform excellently in peroxymonosulfate (PMS)-based advanced oxidation processes (AOPs), for which the generation of reactive oxygen species (ROS) is essential to the degradation of emerging organic pollutants in water. However, the detailed PMS activation mechanisms remain elusive. Density functional theory (DFT) calculation as a powerful approach can overcome the limitations of the experimental studies, providing a molecular-level perspective of catalytic process. This study conducted DFT calculations to clarify the electronic structures and PMS adsorption and activation mechanisms of a series of transition metal single-atom catalysts. According to the DFT study, significant electronic interaction and negative formation energy make nitrogen-doped carbon (N@C) supports suitable for stabilizing metal atoms (Me) to form MeN@C catalysts. As the active site, single metal atom adsorbs the oxygen atoms of PMS by electrostatic and magnetic interactions, and transfer electrons from MeN@C to activate PMS. Different adsorption configurations and the subsequent PMS activation lead to the generation of various ROS, including the SO4•- radical, •OH radical, singlet oxygen (1O2), high-valent metal-oxo species, and surface-activated PMS*. Electron transfer mediated by surface-activated PMS* may dominate in all MeN@C/PMS systems. The generation of free radicals can be difficult for some systems. High-valent metal-oxo species are readily formed by FeN@C/PMS and MnN@C/PMS, whereas 1O2 tends to be produced by CoN@C/PMS, NiN@C/PMS, and CuN@C/PMS. The findings will provide a theoretical basis for the design and synthesis of effective MeN@C catalysts for the AOPs to remove emerging organic pollutants from water and wastewater.